1. Field of the Invention
The present invention relates to the inspection of insulated pipes, vessels and the like, and particularly to a system for the inspection of insulated pipes and vessels using backscattered radiation and X-ray fluorescence.
2. Description of the Related Art
Large insulated pipes are often found in numerous industries, such as in gas producing plants for carrying both liquids and gas, particularly for maintaining fluids under low temperatures, in electrical plants for high temperature applications, and in many other industries where both high and low temperature fluids are utilized. The insulation around the pipes is necessary for maintaining relatively low and high fluid temperatures. The insulation layer on the pipes in these plants, as well as in various other industrial applications, is typically at least several centimeters thick, thus making it extremely difficult to inspect the pipe bodies for corrosion. Plant production must be stopped for interior visual inspection of the pipe walls, and removal of the outer insulation for exterior visual inspection not only requires a great deal of time and expense, but can be detrimental to the pipe itself, since ice forms on the exposed pipe surface for low temperature applications, along with potentially dangerous increases of pressure in the interior, and since heat is lost in high temperature applications. Additionally, such visual inspections of the pipe exterior will not indicate corrosion formed on the interior of the pipe. As noted above, conventional interior inspection would require a shutdown of the plant processes. In addition to visual inspection, other techniques, such as ultrasonic inspection, have been tried, but such methods have typically been found to be difficult to implement due to the inability of the ultrasonic probe to make contact with the pipe or tank wall due to the insulation. Further, ultrasonic methods typically do not work well in the high temperature environments of fluid-carrying pipes and the like.
Although direct radiography allows for inspection of such pipes without the removal of the insulation layer, direct radiography has a number of drawbacks. As illustrated in FIG. 2, in conventional direct radiographic inspection, a radiation source 100 is positioned on one side of the object under inspection and radiographic film or an image plate is positioned opposite the source 100. In the specific application of insulated pipe inspection, the radiation source 100 emits radiation 102, which may be X-rays, gamma rays or the like, which pass through an insulated pipe, which includes a conventional pipe 106 carrying some sort of fluid 112, the pipe 106 being surrounded by an outer annular insulation layer 104. A radiographic film or image plate 110 is placed to the other side of the pipe for imaging corrosion 108 that may be formed on the pipe 106. A wide variety of other techniques involving radiographic imaging have been used, such as insertion of radioisotopes within the pipe for use with an external detector, an internal floating camera, etc. Such methods, though, require complete plant shutdown and are typically highly impractical and expensive to implement.
The attenuation of X-ray and gamma ray radiation is very high in large bodies, such as in the exemplary insulated pipe of FIG. 2. If the object is very large, not enough radiation reaches the film or image plate 110 due to attenuation in the fluid 112 and in the metal wall (typically iron or iron-based materials) wall of the pipe 106. Additionally, as illustrated in FIG. 2, a relatively wide beam must be used, allowing for inspection of all sides of the pipe, which is often not possible for very large pipes or tanks. If a linear accelerator or cyclotron is used as the radiation source, such a wide beam is often impossible to produce. Further, such equipment cannot be used if there is no accessible space available on one side of the object.
Further, due to the use of the single source, all sides of the pipe are imaged at the same time. This often creates confusion about the actual location of corrosion 108, since the image produced on the plate 110 is two-dimensional.
Gamma ray or X-ray backscattering are known techniques for determining metal thickness, such as in measuring the thickness of corroded portions of metal bodies. In backscattered radiation imaging, a gamma ray or X-ray beam is projected incident on the wall of the pipe. Its energy can be selected to be great enough that attenuation in the insulator is insignificant. As gamma rays or X-rays penetrate the pipe, the radiation undergoes attenuation, the radiation intensity decreasing exponentially with wall thickness. The magnitude of attenuation depends on the energy of the incident radiation and the nature of the material. Backscattering takes place from within layers of the wall by Compton interactions. The backscattered radiation undergoes higher attenuation in its path back to the detector or the film, since its energy is lower than that of the primary incident radiation. The radiation will, therefore, undergo double attenuation.
In X-ray fluorescence (XRF) imaging, the incident radiation interacts with the pipe material, followed by emission of XRF radiation. This type of X-ray is characteristic of wall materials. Most pipes and vessels of the type of interest have walls made from iron or iron-based materials. The emitted X-rays have relatively small energies, typically around 7 keV. Additional detectors having high sensitivity for low energy radiation may be used if the first detector is not sensitive enough. It is generally preferable to use a radiation source that emits low energy in order to have a high level of reaction with the object materials. Because of the low energy of the XRF radiation, it is emitted from the surface of the object wall, thus it can image the outer surface of the object. This makes XRF desirable for insulated pipe inspection, since corrosion takes place on the outer surface of the pipe due to moisture trapped under the insulating layer, as well as on the wall body, which causes changes in thickness.
In FIG. 3, a radioactive source 100 emits one or a few well-defined gamma rays. The radiation 102 that is incident on the pipe wall 106 (and passes through insulating layer 104) is collimated by a collimator 114. A portion of the incident radiation 102 will backscatter due to Compton interactions, and a portion will also produce XRF radiation. The backscattered radiation 124 is measured by a gamma ray detector 118 (typically including a spectrometer, such as a NaI (Tl) scintillation detector), while the XRF radiation 120 is measured by a low energy X-ray detector 116, such as a CdTe, Si(Li) or HgI2 detector. Typically, both types of detectors must be utilized, as the NaI (Tl) scintillation detector does not properly detect X-rays and, similarly, the CdTe, Si(Li) or HgI2 detector is ineffective in detecting gamma rays. Further, it can be easily seen that the backscattered radiation 124 is received by detector 118 in a wide variety of angles, rather than being received at a desired angle.
Backscattered radiation, measured at a fixed angle θ, and the XRF each give defined peaks when measured with energy analyzers, such as conventional multichannel analyzers. Counting windows can be selected to measure backscattered radiation peaks and XRF radiation. Single detectors, as illustrated in FIG. 3, though, are limited in their functionality, due to limitations in positioning, fixed degrees of angular measurement, and limited views of only portions of a pipe under inspection. It would be desirable to provide a scanning system capable of constructing an entire pipe wall image. It is further necessary to provide proper imaging hardware and software for converting counts into images using computer imaging programs, such as LabVIEW®, for example, coupled with scanning.
Thus, a system for the inspection and imaging of insulated pipes and vessels using both backscattered gamma radiation and X-ray fluorescence solving the aforementioned problems is desired.